Of Water And Albedo

As usual, there is more to learn in the CERES satellite dataset. I got to thinking of the idea put forth by Lacis 2010. He announced model results claiming that if the only modeled greenhouse gas in the modeled atmosphere were modeled water, the model world would basically evolve to a modeled ice over condition at a modeled -20°C (-4°F). Here is his money graph, showing the evolution of various modeled climate measurements in the first fifty modeled years after removing all modeled GHGs except for modeled water from the modeled atmosphere. See his paper for details.

Now, planetary albedo is what percent of the sunshine is simply reflected back into space. Currently it’s around 30% (orange line, left end, right scale). But how can the planetary albedo be going up in his results by a third, while at the same time the atmospheric water content is reducing by 90%?

Globally the albedo is ruled by clouds. You’d think the variable sea ice and snow on land during winter would matter more to the albedo. But consider how much sunlight there is during a polar winter … the albedo is high up north because of ice and snow, but the weak winter sun may not even come up over the horizon. As a result, ice doesn’t affect the total planetary albedo as much as you might think, much less than clouds, simply because it’s generally not reflecting much sunshine.

So how could albedo be going up when the amount of water in the atmosphere was going through the floor? My intuition, my bad number detector going Hmmm, said no way it could do that—but I realized I didn’t really know that.

So I thought I’d take a look at the data. Remote Sensing Systems (RSS) publishes a gridded dataset of total precipitable water over the ocean. It’s available here as a 3-D NetCDF file (lat/long/time). So I compared that measurement of atmospheric water to the measured albedo of the CERES dataset. The result is below, showing the correlation of total precipitable water (TPW) and total albedo (surface plus cloud). The monthly seasonal variations were removed from the data before analysis. This is the correlation of 17 years of data.

I can’t tell you how much fun it is after laboriously writing the computer code designed to create a new result, hitting run … and then waiting for the image to appear. It’s always a surprise and a joy, new understanding, new intuitions. But I digress …

As you can see, indeed the general pattern is, more water = greater albedo. There are only limited areas of exception to that. This correlation is strongest in the western tropical Pacific, where the ocean is warmest.

So I have to doubt the Lacis result simultaneously claiming much less water and yet greater albedo. Less water = less clouds = less albedo, not more as Lacis claimed.

My next objection to the Lacis result is the precipitous drop in surface temperature. Presumably inter alia it is a result of the great reduction in incoming sunshine due to the fantasized albedo increase. But as shown in the figure above, less water in the air = small albedo, not larger as Lacis claims.

Nor is the temperature drop a result of the loss of atmospheric longwave absorption due to the poorly-named “greenhouse gases” in the atmosphere. There’s an amazing on-line line-by-line atmospheric calculator called MODTRAN. And according to MODTRAN, the loss of all of the GHGs except water would cool the planet by from 6°C to 8°C, with the smaller value in subarctic winter and the larger in the tropics. This is far from enough to take the global average down to minus twenty as claimed.

In addition, there are larger forces at play. Let’s consider three inter-related measurements—ocean sea surface temperature (SST), total precipitable water (TPW), and albedo. All of them rise and fall together. The warmer the sea surface is, the more water there is in the atmosphere, because evaporation increases with temperature.

The warmer the SST, the more water in the atmosphere, the more clouds. The more clouds, the greater the albedo.

Now, think about the effect as this relationship unfolds over time. Warmer ocean waters lead to more atmospheric water leading to greater albedo, which results in less sunshine making it to the surface … which results in cooler ocean waters …

Or we can look at it the other way. Cooler ocean waters lead to less atmospheric water leading to lower albedo, which leads to more sunshine making it to the surface … which results in warmer ocean waters …

How about that for a lovely thermostatic phenomenon? Cools the ocean when it’s warm, warms the ocean when it’s cool!

And that’s the main reason I disbelieve the Lacis model result claiming that the world will go to -20°C. The world is full of many such emergent phenomena that all tend to stabilize the surface temperature. The temperature is not some slavish linear function of the forcing as is generally claimed. It’s far more complex than that.